Discord Provider

[l1n]

Implemented per https://discordapp.com/developers/docs/topics/oauth2.

[ploxiln]

I'd rather the import path not be renamed from github.com/bitly/oauth2_proxy to github.com/ploxiln/oauth2_proxyin this PR. It may be a bit awkward, but you can clone my repo to$GOPATH/src/github.com/bitly/oauth2_proxy`, or if you already have the bitly repo there you can add my repo as another remote and check out my master branch, etc. I'll probably do the renaming soon - but separately.

Other than that, there's just some minor whitespace/formatting issues.

[l1n]

Fair enough, reverting that commit 👍 I didn't mean to add it in this pull request since it wasn't part of the Discord provider.

[ploxiln]

Thanks!

(I pushed up one last gofmt tweak ... the trick is gofmt -w providers/*.go)

[l1n]

I apologize for not speaking Go too natively, my last Go project was a year or so ago 😅 thank you for your help

[ploxiln]

looks like "connections" is not needed

[ploxiln]

this and some other little refinements could be in a follow-up pull request if someone cares enough :)

(e.g. removing parts of DiscordUserInfo which are unused, maybe checking Verified property ...)

[l1n]

It isn't needed right now, but I'd like to extend from the user and email that are by default provided to the proxied host at some point: similar to the way that mod_shib provides displayName https://tcg.stanford.edu/?p=131